Excimer lasers are well known. An important use of excimer lasers is to provide the light source for integrated circuit lithography. The type of excimer laser currently being supplied in substantial numbers for integrated circuit lithography is the KrF laser which produces ultraviolet light at a wavelength of 248 nm. A similar excimer laser, the ArF laser, provides ultraviolet light at 193 nm, and an F.sub.2 laser operates at 157 nm. These lasers typically operate in a pulse mode at pulse rates such as 1,000 Hz. The laser beam is produced in a laser chamber containing a gain medium created by the discharge through a laser gas between two elongated electrodes of about 28 inches in length and separated by about 5/8 inch. The discharge is produced by imposing a high voltage such as about 20,000 volts across the electrodes. For the KrF laser, the laser gas is typically about 1% krypton, 0.1% fluorine and about 99% neon. For the ArF laser the gas is typically about 3 to 4% argon, 0.1% fluorine and 96 to 97% neon. The F2 laser is about 0.15% F.sub.2 and the rest He. In both cases in order to achieve high pulse rates of about 1,000 Hz, the gas must be circulated between the electrodes at speeds of about 500 to 1,000 inches per second.
Fluorine is the most reactive element known and it becomes even more reactive when ionized during the electric discharge. Special care must be exercised to utilize in these laser chambers materials such as nickel coated aluminum which are reasonably compatible with fluorine. Further, laser chambers are pretreated with fluorine to create passification layers on the inside of the laser chamber walls. However, even with this special care, fluorine will react with the walls and other laser components producing metal fluoride contaminants and resulting in a relatively regular depletion of the fluorine gas. The rates of depletion are dependent on many factors, but for a given laser at a particular time in its useful life, the rates of depletion depend primarily on the pulse rate and load factor if the laser is operating. If the laser is not operating, the depletion rate is substantially reduced. The rate of depletion is further reduced if the gas is not being circulated. To make up for this depletion, new fluorine is typically injected at intervals of about 1 to 3 hours. Rather than inject pure fluorine it is a typical practice to inject into KrF lasers a mixture of 1% fluorine, 1% krypton and 98% neon. For example, in a specifically treated high quality 1000 Hz KrF excimer laser used for lithography, the quantity of its fluorine, krypton, neon mixture injected to compensate for the fluorine depletion varies from about 5 scc per hour when the laser is not operating and the laser gas is not being circulated to about 180 scc per hour when the laser is running continuously at 1000 Hz. The typical injection rate is about 10 scc per hour when the chamber fan is circulating the laser gas, but the laser is not firing.
The unit "scc" refers to "standard cubic centimeters". Other commonly used units for describing quantities of fluorine in a particular volume are percent (%) fluorine, parts per million and kilo Pascals; the latter term sometimes refers to the partial pressure of the fluorine gas mixture. (This is because the amount of fluorine injected into a laser chamber is typically determined (directly or indirectly) by the measured chamber pressure increase while the 1% fluorine gas mixture is being injected.) A 195 scc per hour injection rate of the 1% fluorine mixture would correspond to a depletion in the fluorine concentration over 2 hours from about 0.10 percent to about 0.087 percent. The actual quantity of fluorine depleted in two hours as measured in grams of pure fluorine would be about 17 milligrams during the two hour period corresponding to the above 320 scc/hour injection rate (i.e., 390 scc of the 1% fluorine mixture injected at two-hour intervals) of the fluorine gas mixture.
For integrated circuit lithography a typical mode of operation requires laser pulses of constant pulse energy such as 10 mJ/pulse at about 1000 Hz which are applied to wafers in bursts such as about 300 pulses (with a duration of about 300 milliseconds) with a dead time of a fraction of a second to a few seconds between bursts. Modes of operation may be continuous 24 hours per day, seven days per week for several months, with scheduled down time for maintenance and repairs of, for example, 8 hours once per week or once every two weeks. Therefore, these lasers must be very reliable and substantially trouble-free.
In typical KrF and ArF excimer lasers used for lithography, high quality reproducible pulses with desired pulse energies of about 10 mJ/pulse for KrF and 5 mJ/pulse for ArF may be obtained over a substantial range of fluorine concentration from about 0.08 percent to about 0.12 percent. Over the normal laser operating range the discharge voltage required to produce the desired pulse energy increases as the fluorine concentration decreases (assuming other laser parameters remain approximately constant). FIG. 1 shows the typical relationship between discharge voltage and fluorine concentration for constant pulse energy of 10 mJ and 14 mJ. The discharge voltage in the range of 15 kv to 20 kv is typically controlled by a feedback system which calculates a charging voltage (in the range of about 550 volts to 800 volts) needed to produce (in a pulse compression-amplification circuit) the needed discharge voltage which is needed to produce the desired laser pulse energy. This feedback circuit therefore sends a "command voltage" signal a power supply to provide charging voltage pulses.
Prior art techniques typically utilize the relationship between discharge voltage and fluorine concentration to maintain constant pulse energy despite the continuous depletion of fluorine. The discharge voltage of prior art excimer lasers can be changed very quickly and accurately and can be controlled with electronic feedback to maintain constant pulse energy. Accurate and precise control of fluorine concentration in the past has proven difficult. Therefore, in typical prior art KrF and ArF laser systems, the fluorine concentration is allowed to decrease for periods of about 1 to 4 or 5 hours while the discharge voltage is adjusted by a feedback control system to maintain constant pulse energy output. Periodically at intervals of about 1 to a few hours, fluorine is injected during short injection periods of a few seconds. Thus, in normal operations fluorine concentration gradually decreases from (for example) about 0.10 percent to about 0.09 percent over a period of about 1 to a few hours while the charging voltage is increased over the same period from, for example, about 600 volts to about 640 volts. The injection of fluorine at the end of the 1 to a few hour period (when the voltage has drifted up to about 640 volts) brings the fluorine concentration back to about 0.10 percent and the feedback control (maintaining constant pulse energy) automatically reduces the voltage back to 600 volts. This basic process is typically repeated for several days. Injections are typically performed automatically as controlled by a controller based on specially crafted control algorithms. As shown in FIG. 2, prior art excimer lasers typically divert a portion of the chamber gas flow through a metal fluoride trap to remove contamination. Laser beam 2 is produced in a gain medium between electrodes 4 (only the top electrode is shown in FIG. 2) in chamber 6 in a resonance cavity defined by line narrowing module 8 and output coupler 10. Laser gas is circulated between the electrodes 4 by tangential blower 12. A small portion of the circulating flow is extracted at port 14 downstream of blower 12 and directed through metal fluoride trap 16 and clean gas is circulated back into the chamber through window housings 18 and 20 to keep the windows free of laser debris. A very small portion of each laser pulse is sampled by beam splitter 22 and pulse energy monitor 24, and in an extremely fast feedback control loop, controller 26 controls the electrode discharge voltage to maintain pulse energy within a desired range by regulating a high voltage charging circuit 28 which provides charging current to voltage compression and amplification circuit 30 which in turn provides very high voltage electrical pulses across electrodes 4. Over a longer term controller 26 through gas controller 27 also controls the fluorine concentration in the chamber 6 by regulating fluorine injections at control valve 32. Special control algorithms will periodically inject predetermined quantities of fluorine. These injections may be called for when the high voltage has increased to a predetermined limit or injection may be made after a predetermined number of pulses such as 3 million pulses or after the passage of a predetermined period of time (such as six hours with no lasing) or a combination of pulses, time and voltage. Typically two gas supplies are available. A typical fluorine source 34 for a KrF laser is 1% F.sub.2, 1% Kr and 98% Ne. A buffer gas source 36 of 1% Kr and 99% Ne may also be tapped by controller 26 through valve 38 when providing an initial or a refill of the chamber or if the F.sub.2 concentrations for some reason gets too high. It Laser gas may be exhausted through valve 40 and the chamber may be drawn down to a vacuum by vacuum pump 42. Exhaust gas is cleaned of F.sub.2 by F.sub.2 trap 44. FIG. 3 shows the results of the prior art fluorine injection techniques discussed above. The voltage values represent average values of control voltage commands and indirectly represent average values of charging voltage. Since contamination gradually builds up in the laser gas over a period of several days, it is usually desirable to replace substantially all of the gas in the laser with new laser gas at intervals of about 5-10 days.
The above-described prior art technique is effectively used today to provide long term reliable operation of these excimer lasers in a manufacturing environment. However, several laser parameters, such as bandwidth, beam profile and wavelength stability, are adversely affected by the substantial swings in the discharge voltage and fluorine concentration.
A substantial number of techniques have been proposed and patented for measuring and controlling the fluorine concentration in excimer lasers to within more narrow limits than those provided under the above described prior art technique. These techniques have generally not been commercially pursued. Prior art commercial excimer lasers typically do not have a fluorine monitor. A need for a good, inexpensive, reliable, real-time fluorine monitor has been recognized for a long time.
Techniques for measuring trace gas concentrations with light beams are well known. One such technique uses a photo detector to determine the absorption of a beam as it passes through an absorption cell. Another technique well known since it was first discovered by Alexander Graham Bell involves the creation of sound waves in an absorption cell with an intensely modulated light beam. See Optimal Optoacoustic Detector Design, Lars-Goran Rosengren, Applied Optics Vol. 14, No. 8/August 1975 and Brewsters Window and Windowless Resonance Spectrophones for Intercavity Operations, R. Gerlach and N. M. Amer, Appl. Phys. 23, 319-326 (1980).
What is needed is a better system and method for dealing with the issue of fluorine depletion in KrF and ArF excimer lasers.